This post was written by a fan of moldy bugs who is a graduate student in Entomology at Cornell.

The Cinderella story of biological control is the introduction of the fungus Entomophaga maimaga (Zygomycota: Entomophthorales) to control gypsy moth caterpillars in northeastern North America. The gypsy moth became established here in 1869, and its fuzzy caterpillars eat the leaves of a wide range of plants and trees (though they prefer oaks). The gypsy moth quickly became a pest and began to cause serious forest defoliation as populations reached extraordinarily high densities in the absence of serious natural predators (aside from generalist insectivores). In 1910 and 1911 fungal pathogens were released, one of which was E. maimaiga, but no resulting infections were found in the larval population and hope for a fungal solution was given up as a lost cause.

Seventy years later, scientists decided to try again, and E. maimaiga was released again. Just as before, however, no lasting infection was found in the field. Then, in southwestern Connecticut in 1989, large numbers of Gypsy moth larvae were found to have succumbed to a fungal pathogen, specifically E. maimaiga1. The fungus has been seen most years since then, and is has spread through northeastern gypsy moth populations. How could this fungal pathogen have taken so long to establish itself? As much as scientists and biological control advocates would love to take the credit for this introduction, genetic and geographic evidence point to the fact that this new epizootic (the animal equivalent of an epidemic) was caused by an accidental introduction of Entomophaga maimaiga. Whoops.2

Here’s the part of the Entomophaga maimaiga life cycle that’s visible to the naked eye. This is also the part that makes it a potent biological control. The time lapse movie below is courtesy of James Reilly.

Time lapse video of our fungus, Entomophaga maimaiga, erupting from the corpse of its host to shoot spores all over the place. Movie by James Reilly.

What you’re seeing here are the late stages of infection. The fungus has eaten away the inside of the caterpillar and will now send out white structures called conidiophores, from which its spores are shot into the air. These spores (asexual conidia) can now infect other caterpillars. If it fails to find a host, a conidium can germinate to make and shoot off a smaller replicate of itself (spiffy!). Spore dispersal allows for direct infection of other caterpillars within the area, but conidia are short-lived and only capable of surviving for a few days in the environment. They’re hoping to land on a caterpillar. From there they’ll grow through the cuticle and start eating up its tissues (think Invasion of the Body Snatchers). Once it’s finished its gypsy moth caterpillar (and this fungus eats nothing else), the fungus bursts back out to produce a new crop of spores. In the video we see the fully colonized corpse shrink as its biomass is converted into fungus. For the pyrotechnic finale, a white dust of spores appears–each spore is independently shot from its mother cell. In the second half of the video, we see the same thing at higher magnification. The conidiophores emerge like small, gooey cauliflowers, and become sprinkled with salt (conidia) that are then shot off. The conidia then accumulate on the hairs like white crystals. By the time we’re done, there’s virtually no fungal biomass left in the caterpillar; it’s all been shot away.

How does this fungus overwinter if these spores are ephemeral and the supply of caterpillars runs out? Towards the end of the season, or towards the end of the larval stage of the caterpillar, E. maimaiga produces hyphal swellings within the insect’s hemocoel (space in the insect that the blood or hemolymph sloshes around in). These swellings develop a thick wall and become asexual resting spores. They can survive for years in the soil litter (where most caterpillar cadavers end up). When presented with favorable conditions, they can germinate to make and shoot off conidia of their own, thus starting up the cycle of infection once again.2

Entomophaga maimaiga is perhaps one of the most successful fungal biological control agents. It belongs to the order Entomophthorales, many members of which are insect pathogens, including our fly-killing friend Furia ithacensis. Entomophaga maimaiga can effectively control outbreak populations of the gypsy moths and has spread to cover most of area inhabited by these moths. Only at the edge of gypsy moth spread is this fungus not present.

If you’re confused about the difference between gypsy moth and northern tent caterpillars, you’re not alone. Here are some native forest tent caterpillars (courtesy Liz Castro), and here are some gypsy moth caterpillars (courtesy of taryn *). What’s the collective term for a group of caterpillars? An army.

About

Most people don't pay much attention to fungi, which include things like mushrooms, molds, yeasts, and mildews. Here at Cornell we think they're pretty fascinating. In fact, even the most disgusting foot diseases and moldy strawberries are dear to our hearts. We'd like to talk to you about fungi, so that like us, you too can tell gross stories at the dinner table. Afterwards, maybe you'll notice some things you would have overlooked before, and we think this could be good for the planet.